"Welcome to your first day, we will not go away!" marchers in Washington chanted.
The rally featured speeches from women's rights activist Gloria Steinem, Planned Parenthood president Cecile Richards, actress Ashley Judd and director Michael Moore among others.
Many of the women came wearing pink, pointy-eared "pussyhats" to mock the new president. Plenty of men joined in, too, contributing to surprising numbers everywhere from New York, Philadelphia, Chicago and Los Angeles to Mexico City, Paris, Berlin, London, Prague and Sydney.
A group of largely women senators and other politicians took the stage together at one point, including Kirsten Gillibrand, Claire McCaskill, and newly elected Senators Kamala Harris and Tammy Duckworth, who addressed the group as did Rep. Maxine Waters. Rep. Debbie Wasserman Schultz and Sen. Cory Booker were also on stage.
The Washington rally alone attracted over 500,000 people according to city officials - apparently more than Trump's inauguration drew on Friday. It was easily one of the biggest demonstrations in the city's history, and as night fell, not a single arrest was reported.
The international outpouring served to underscore the degree to which Trump has unsettled people in both hemispheres.
"We march today for the moral core of this nation, against which our new president is waging a war," actress America Ferrera told the Washington crowd. "Our dignity, our character, our rights have all been under attack, and a platform of hate and division assumed power yesterday. But the president is not America. ... We are America, and we are here to stay."
Turnout in the capital was so heavy that the designated march route alongside the National Mall was impassable. Protesters were told to make their way to the Ellipse near the White House by way of other streets, triggering a chaotic scene that snarled downtown Washington. Long after the program had ended, groups of demonstrators were still marching and chanting in different parts of the city.
Steinem thanked the crowd for showing up en masse, declaring, "We have people power and we will use it."
"Thank you for understanding that sometimes we have to put our bodies where our beliefs are. Sometimes pressing send is not enough," she said to the crowd, many of whom wore bright pink knitted hats.
Steinem suggested that the size and energy of today's gathering in Washington was a positive outgrowth of Trump's election and inauguration.
"This is the upside of the downside. This is an outpouring of energy and true democracy like I have never seen in my very long life. It is wide in age, it is deep in diversity," Steinem said.
She praised "our great leaders" Barack and Michelle Obama, Sen. Bernie Sanders, and Hillary Clinton "who told the whole world that women's rights are human rights and human rights are women's rights," quoting Clinton's speech at a United Nations conference in 1995.
Steinem then turned her focus to President Trump.
The new president's "Twitter finger must not become a trigger finger," she said.
Shortly before Steinem took the podium, actress America Ferrera was one of the first speakers at the rally ahead of the march, calling for supporters to "fight, oppose" the Trump administration.
"Marchers, make no mistake. We are -- every single one of us -- under attack. Our safety and freedoms are on the chopping block," she said.
Some of the homemade signs at the rally were related to Hillary Clinton, Trump's campaign rival. One read "Still With Her" using a play on Clinton's campaign slogan, and another read "Lock Him Up," playing on a chant that some Trump supporters directed at Clinton during the campaign. Several "Stronger Together" posters from the Clinton campaign were spotted as well.
While she wasn't there in person, Clinton gave her support via Twitter.
Planned Parenthood president Cecile Richards, actress Ashley Judd and D.C. mayor Muriel Bowser addressed the crowd, as did director Michael Moore who railed against President Trump, telling marchers to take action against his administration.
"We have to get busy folks. We've got our work cut out for us," Moore said.
The rally and ensuing march come the day after Donald Trump was sworn in as the 45th president of the United States.
The inauguration drew hundreds of thousands of attendees on Friday in spite of wet weather, and today's drier forecast made the travel to Washington easier for today's marchers.
A number of high-profile speakers addressed those at the rally, including Gloria Steinem, Planned Parenthood's president Cecile Roberts, and director Michael Moore.
White House press secretary Sean Spicer had no comment on the march except to note that there were no firm numbers for turnout because the National Park Service no longer provides crowd estimates.
Around the world, women brandished signs with slogans such as "Women won't back down" and "Less fear more love." They decried Trump's stand on such issues as abortion, health care, diversity and climate change. And they branded him a sexist, a bully, a bigot and more.
"We want a leader, not a creepy tweeter," some marchers chanted in Washington.